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Warm Weather Reignites Disease Ahead of T2 Spray

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A resurgence of septoria and yellow rust in wheat crops has heightened the need for a robust and well-timed flag leaf spray, leading agronomy firm Hutchinsons warns.

Although cool April conditions held back disease pressure in many crops, the warmer start to May has reignited septoria and rust concerns. Even crops that appear “clean” are at risk as the cool April has extended the latent period between septoria infection and onset of visible symptoms, agronomists warn.

Problems are most pronounced where earlier T0 and T1 sprays have been disrupted due to poor weather, putting more emphasis on the T2, says technical development director Dave Ellerton.

“Septoria is always the main threat at T2 and cannot be allowed to establish on the flag leaf.”

“But we’ve also seen yellow rust flare up again and affect varieties you would not expect it to be a problem in. This may be due to sprays being delayed, but it is a concern that aggressive new rust races are also developing, potentially infecting crops in cooler conditions and later in the season.”

Recommended List resistance ratings should only be used as guidance and are not definitive, Dr Ellerton warns. “Keep a close eye on every variety and be prepared to knock disease out rapidly. We’ve still got a good range of chemistry available to do that at the flag leaf timing.”

T2 options and timing

Applying fungicides with strong curative action is essential given the high disease pressure and Dr Ellerton says that means basing the T2 spray around robust SDHI chemistry.

Where rust risk is high, a rust-active triazole such as epoxiconazole, tebuconazole or metconazole should be included to bolster curative and protectant activity, he says. “Strobilurins also offer good persistence against rust but have less curative ability.”

Timing the T2 spray accurately will be trickier this season given the wide variation in growth stages and rapid development of crops making up growth in warmer conditions, he notes. Crops typically ranged from stem extension (GS 30) to GS 33 (third node detectable) at the time of writing (see regional roundup below).

“Once the flag leaf is at least three-quarters to fully emerged, that’s when you have to apply the T2, even if that’s only 10-14 days after the T1. You cannot afford to leave the flag leaf exposed and let disease in as you’ll struggle to knock it out. If you end up trying to chase septoria, you’re in big trouble.”

A well-timed T2 spray also allows growers to focus on the key ear diseases at T3 rather than having to chase foliar control, Dr Ellerton adds. “Weather at ear emergence dictates T3 choice. Cool and wet favours microdochium development, so prothioconazole is the better option. Warm, wet conditions favour fusarium, in which case prothioconazole, tebuconazole and metconazole are all effective.”
He advises growers use the AHDB fusarium risk assessment http://cereals.ahdb.org.uk/mycotoxins

resurgence of septoria

Regional round-up

South east – James Short, Kent

Wheat crops in Kent tend to be some of the earliest in the country, but even here progress is 7-10 days behind normal, according to James Short.
“There’s an incredible range of growth stages from 30 to 33 across different varieties, soil types and drilling dates. Most crops have received their T1 fungicide and the weather over the next fortnight will dictate speed of crop and disease development ahead of the flag leaf spray.”

Although disease pressure in some crops appears relatively low, he believes infection is present within canopies and symptoms could easily flare up given the right conditions. Septoria, yellow and brown rust are already affecting older varieties with weaker disease ratings, but all crops should be regarded as being at high risk, he says.

Mr Short favours robust rates of fluxapyroxad and epoxiconazole + metconazole at T2, with doses tweaked according to disease pressure.

Southwest – Amie Hunter, Cornwall

The majority of wheat crops in the southwest have reached GS 33 and should receive the flag leaf spray by the third week in May, depending on the weather, Amie Hunter says.

“Growth is around a week behind normal, but crops should even-up in the warmer weather. Forward wheats have a lot of potential, but disease pressure is high.”

With very little yellow rust in evidence, apart from a few cases in the variety Reflection, septoria is the main concern, she continues. “There’s a lot on lower leaves and I suspect there’s a lot in the crop that’s still in the latent phase and not yet showing symptoms.”

She too advises a robust 1.25litres/ha dose of fluxapyroxad as the T2 foundation, together with a triazole-based partner such as epoxiconazole + metconazole, or epoxiconazole + fenpropimorph if mildew is an issue.

Barley crops also face high disease pressure, with rhynchosporium being the main concern and some ramularia starting to show in crops close to the coast, she notes.

Scotland – Cameron Ferguson, Ayrshire

Ayrshire agronomist Cameron Ferguson is battling to keep septoria at bay in forward wheats at GS 32, while backward crops are barely past late-tillering after an unseasonably cold, wet April that has played havoc with early fungicides.

“Some growers have really struggled to find an opportunity to apply the T0 and for many it is now stretching into the T1 window. Most T1s should be applied in the next week, providing the weather settles down, which puts the T2 spray around the end of May.

“After the mildest winter on record it’s already become a very difficult season and one that’s made worse by the tight margins across livestock and arable sectors.”

He urges growers to resist the temptation to cut fungicide costs too far and run the risk of letting disease get established. Where disease pressure is high, especially if the T0 has been missed, he recommends supporting epoxiconazole and chlorothalonil-based products at T1 with the stronger curative action offered by SDHIs such as penthiopyrad or fluxapyroxad.

Mr Ferguson also says rhynchosporium pressure in barley is higher than normal and advises growers to be vigilant in the run up to the T2 spray.
Prothioconazole is the most effective triazole in barley, but should be bolstered with penthiopyrad or fluxapyroxad where pressure is high. “Epoxiconazole + isopyrazam is another option that works well.”